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in an unchanging manner then the rains fall, the lettuce grows and the rabbits create little rabbits in numbers that exactly match the food value of the growing lettuce.

In the small this illustrates that the extent of an ecosystem can be established along a number of axes. In our little hypothetical ecosystem, there are a number of boundary characteristics; for example, the amount of land suitable for growing lettuce, climatic conditions including rainfall rates, propagation rate of rabbits, total number of rabbits, total amount of lettuce, and the amount of water produced by the spring. We can probably identify quite a number more, but these certainly provide a good start. Thus, we see that by identifying various attributes of a specific ecosystem, that ecosystem can be characterized and thereby allow analysis, and some understanding of the interactions that occur within. Consequently, we consider that such attributes constitute the effective boundaries of the ecosystem.

Extrapolating from our very simple ecosystem to a more complex realistic one, we see that a boundary can be simple and straightforward in identification and understanding, or it may be quite abstract. In some instances, a purely physical boundary may delineate an ecosystem, or at least some aspect of an ecosystem, while in others a virtual boundary formed by a strong gradient of change in a critical characteristic may occur. The boundaries of an ecosystem might form a complex container of interactions; one part perhaps providing a barrier to passage of certain species while a different characteristic (that is, a different boundary) effects a barrier for other species.

We view an ecosystem as a piecewise closed environment, recognizing that it is unusual to be able to characterize a completely closed environment for the full range of species found within, unless, of course, one simply considers the single ecosystem of the earth and the nearby celestial environment that bears directly upon it. Looking at ecosystems in the small, it is rather typical to find some species that can cross the boundaries of ecosystems and move rather freely among many ecosystems. While diverse ecosystems can perhaps be characterized without significant regard to such migratory species, in fact the migratory species depend on a variety of ecosystems for various aspects of their lifecycle. This constitutes a hybrid variant of an ecosystem; one that can be compartmentalized into significantly different subsections. Consider some of the unique wetlands ecosystems that exist along the coastlines of landmasses throughout the world, and specifically those of the Pacific Northwest and the Gulf Coast regions of the United States.

In the wetlands of the Columbia River basin, and among the estuaries of the Columbia River system, anadromous species such as the Pacific Salmon are born and spend their adolescent lives in the fresh waters of the wetlands. Passing adolescence, they migrate to the deep oceans where they reach sexual maturity and from which they subsequently return to the wetlands from whence they came. Here, they can spawn and create the next generation of the species. Thus, two very different environments can be viewed as merged into a single super-ecosystem within which the Pacific Salmon goes through its complete lifecycle. In a similar vein, but a very different manner, the whooping crane is hatched and nurtured in the wetlands along the upper Gulf coast of Texas. The wetlands there provide the fertile feeding grounds and refuge from many predators that allow the nesting, hatching and early development of the next generation of the species. Year after year, more than half of the entire whooping crane population of the earth makes its trek to the Texas coast and winters there. In the spring, they migrate to their summer feeding grounds in northwestern Canada. So, the whooping cranes have adapted to one ecosystem for a part of their lifecycle and a different ecosystem for a different part of their lifecycle. While at the present time, whooping cranes are so rare that they don’t provide a significant drain on the resources of either ecosystem, their adaptation lessens their drain on each ecosystem, and affects their relationship to predators.

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